Monolithic frequency synthesizers are well known and have been used as key building blocks in a wide range of applications. In communications, they generate a GHz level frequency carrier either through an inductor-capacitor (LC)-tank oscillator or a ring oscillator (RO), depending on the application's set of requirements and integrated circuit (IC) implementation tradeoffs.
Ring oscillators typically occupy a small silicon area and offer wide tuning range, thus making them flexible and inexpensive at any CMOS node, which is highly desirable in wireline systems. Their intrinsically poor phase noise (PN) and high sensitivity to supply voltage variations (i.e. frequency pushing), however, limit their use and often require either very complex calibration or high power-supply ripple rejection ratio (PSRR) low drop out regulators (LDOs) that employ large capacitors, thus effectively reducing their area advantage.
On the other hand, LC-tank oscillators inherently feature much lower phase noise and frequency pushing. Their power efficiency is superior due to a high quality (Q)-factor of their inductor. The frequency pushing is mostly related to a voltage dependence of their active devices' parasitic capacitances. Some classes of LC-tank oscillators, such as class-F or class-C, with an inductor or a transformer, exhibit small frequency pushing since their active devices require biasing networks of zero dc current, thus promoting effective filtering of dynamic variations (i.e. noise, ripple) on the supply line VDD.
Drawbacks of LC-tank oscillators, however, include (1) their large size and (2) narrow tuning range. There are a few cases in prior art literature where small inductors were designed (1) by stacking metal layers in a vertical solenoid fashion or (2) by plenary shrinking the inductor. In the former case, the area obtained was extremely small, but its tuning range of only 5-10% was impractical in the face of process variations. In the latter case, the large number of turns caused the inductor to be very sensitive to its surroundings, requiring the same techniques used for large inductors to avoid Q-factor deterioration, such as complete inductor isolation, which is not permitted anymore in advanced technologies.
Although wide tuning range can be achieved with high-Q oscillators, as in dual mode, similar techniques employed to conventional LC-tank oscillators impose severe constrains on area and, consequently, coupling to nearby circuits. A switched-inductor topology has been proposed which was possible due to a combination with a pair of analog varactors as coarse/fine tuning banks, but with extremely high sensitivity (worst case on the order of 2.5 GHz/V), requiring a very stable control voltage (and thus a large capacitor) that would reduce any size advantage.
Based on the above observations of prior art attempts at compact oscillators, the desired oscillator would combine superior PN and frequency pushing of an LC-tank oscillator with low area and wide tuning range (i.e. 2:1 tuning range is required to generate an arbitrarily lower frequency through an integer division) of an RO. Furthermore, to exploit the scaling of CMOS technology, a digital manner of frequency tuning would be desired.